Open counter for low energy electron detection with suppressed background noise

ABSTRACT

A photoelectron counter in an open detection chamber in which photoelectrons emitted from a solid surface by a photon irradiation energy are counted, is arranged so as to suppress background noise which produces a false count rate due to photoelectrons which are emitted from solid surfaces outside the subject when scattered rays have reached and irradiated the same surface. The counter uses one or more of options including a film or coating formed with a thickness which is less than several μm thickness on parts which are subject to scattered rays incident from the subject, a ray screen for interrupting scattered rays from entering into the detection chamber and/or a center guard provided at the front part of the detector to enable nearby measurement and while interrupting scattered rays.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a device for detecting photoelectrons in airwhich will be emitted from solid surfaces by a work function of photonirradiation energy using different photoelectric work functions ofrespective solids in place of using function of kinetic energy,especially to a device for suppressing background noise as adds falsecounting rate to true counting rate measured on a detection sample,which will be produced when photoelectrons are emitted by scatteredreflection from the detection sample.

(2) Description of the Prior Art

An open counter for photoelectron detection was disclosed in JapanesePatent Open Publication No. 60-262005 (application No. 59-118,818 filed1984), in contrast with photo electron detection in an ultra-highvacuum. Such a counter was also disclosed by the inventor in the paper,Japanese Journal of Applied Physics Vol. 24 (1985), Supplement 24-4. pp.284-288.

The photoelectron detection chamber 1 of the device partially shown inFIG. 1A comprises a grid G6 for quenching gas discharge, a grid G7 forsuppressing and neutralizing positive ions and an anode A5 using aloop-shaped tungsten filament, wherein the grid G6, G7 and anode A5 arerespectively supplied with +100 V, +80 V and +3.5 kV with respect to thechamber 1 grounded as a cathode. The electrons were emitted from thesubject upon irradiation by photons and accelerated by two grids tobecome attached to air or O₂ molecules to form negative ions in theatmosphere of the detection chamber. Near the anode A5 electrons weredetached from negative ions and caused a gas discharge which was inducedby high positive electric fields applied to the anode (+3.5 kV). Bydetecting a reduction in the high voltage on the anode due to an initialdischarge, quenching was carried out by supplying a positive squarepulse (+300 V in amplitude and of about 3 msec in width Te) to thequenching grid G6. Some of the positive ions produced around the anodecould pass through the quenching grid G6 during a discharge followed byquenching. To neutralize these positive ions, -30 V was supplied to thesuppressor grid G7. Such a series of procedures prevented successive andcontinuous discharges, and enabled electrons in an atmosphere to becounted without either self-quenching or internal quenching. Afterpositive ions were neutralized, the grids G6 and G7 are turned to theinitial voltages referred to FIG. 1B.

Secondary discharge incurred by positive ions was avoided by repeatingthe above procedure for stably counting the rate of photoelectrons inair.

When a ray source spectroscope in FIG. 1A which can change thewave-length of supplied irradiation energy from a lower value (longerwavelength) to a higher value, a certain amount of energy causesphotoelectron emission due to the photoelectric effect. The countingrate (Hz) of photoelectrons per second and ray irradiation energy (hv)have the following relationship:

(Hz)^(n) αhv, wherein n=0.4˜1 usually, n=1/2 for metal.

Photoelectron emission energy is given by a work function and the valueof the work function is different for different kinds of substances. Inthe case of an oxide layer on silicon, the work function of the oxide islarger than that of silicon so that presence of the oxide layer reducesthe amount of photoelectrons emitted from that emitted from silicon.When the photoelectrons are given an irradiation energy which is largerthan the work function of silicon, the counting rate of photoelectrons Nis given as follows:

log N=N₀ -T/2.3λ, wherein T is the thickness of the layer: N₀ is thecounting rate at a thickness of zero: λ is the mean free path in theoxide layer. Generally, other substances have equations similar to this.A value of N of 350˜1 Hz was confirmed in response to thicknesses from0˜140 A°. In the above noted paper, compensation was also disclosed foratmospheric pressure, temperature and humidity changes. However, thereis no suggestion in the abovenoted paper as to the problem of scatteredray irradiation causing a false increase in the photoelectron countingrate as background noise. It is impossible to obtain an exact countingrate of photoelectrons using suitable compensation equations unless thebackground noise of scattered ray irradiation was suppressed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a device for countingphotoelectrons in air with a suppressed background noise, which is usedto obtain solid surface information regarding friction, abrasion,mechanical deformation, oxidation, catalytic reactions, radiationdamage, contamination, thickness of oxidation and work functions, whilephotoelectrons emitted from peripheral solid surfaces by scattered rayirradiation from the subject cause an erroneous counting rate increaseas background noise.

Another object of the present invention is to provide an open counterfor low energy electron detection using a ray screen for suppressingemission of extraneous photoelectrons so as to reduce the backgroundnoise which cause the false counts to

Another object of the present invention is to provide an open device forcounting photoelectrons from the subject using a thin layer formed ongrids and/or parts with a thickness of less than several μm so as tothereby to suppress the background noise.

Still another object of the present invention is to provide a device forcounting low energy electrons in air using a center guard structure toenable nearby measurement and background noise suppression against thesubject.

Background noise may occur so much as to make it impossible to make anexact count of photoelectrons even if a compensation factor wascalculated. Referring to FIG. 1A, when a surface of a specimen 2 is notmirror-polished, a ultra-violet beam 3 is radiated to and scattered fromthe subject surface, so that the scattered rays 10 enter from the lowenergy electron entrance gate 9 to the inside space. The scattered rayswhich irradiate inside parts, especially the grids G6 and G7 cause themto emit low energy electrons.

FIG. 2 shows graph of background noise of which the counting rate (Hz)of photoelectrons was plotted against the wavelength of irradiating rayswhen the open counter was receiving rays scattered from the subject, andthe subject was an A1 plate. Curves S1 and S2 are respectively the totalcounting rate (Hz) of emitted photoelectrons, with S1 be obtained from aspecimen polished in the manner illustrated in FIG. 3 (a) and S2obtained from another specimen as illustrated in FIG. 3(b).

Curves N1 and N2 are respectively the counting rate (Hz) of emittedphotoelectrons when the grid G7 was charged to -30 V so as to therebyblock incoming low energy electrons from the speciman 2. Detectedelectrons were emitted from the grids G6 and G7 by irradiation of raysscattered from the specimen 2. Moreover, such background noise countingrates N1 and N2 have peak values near the wavelength 240 nm,respectively. N1 and N2 are respectively observed from the same specimenof FIGS. 3(a) or 3(b) as rated above for S1 or S2. The exact countingrate of low energy electrons which are emitted from the subject will beobtained by the calculation of (S1-N1) or (S2-N2); however, it isimpossible to correct the total counting rate (Hz) if the backgroundnoise were large. It is still unknown whether an increase inphotoelectrons depends on the difference of scattered rays or thethickness changes of the oxide layer as long as the background noiseexisted. The anode A5 is supplied with a voltage Va by a high voltagesource 16; the grid G6 is supplied with a voltage Vg1 by a first pulsegenerator 18; the grid G7 is supplied with a voltage Vg2 by a secondpulse generator 19; the ray screen 14 is supplied with a voltage Vc by aray screen voltage source 15; and a reduced voltage lower than Va isinput to a counting circuit 20 through an amplifier 17 and is correctedby a compensation circuit 21 using a computer to supply the countingrate (Hz) to a display 22.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a known open counter for low energy electron detection insectional view together with a ray source device and an operationelectric circuit, to which a ray screen 14 is applied according to thepresent invention;

FIG. 1B is an electric signal chart for obtaining counting rate (Hz) ofphotoelectrons by controlling the counter operation;

FIG. 2 is a graph of background noise;

FIG. 3 (a) and FIG. 3 (b) are sample specimens which were polishedvertically or transversely for changing the direction of scatteringrays;

FIGS. 4 and 5 are respectively a ray screen in plane view;

FIG. 6 (a) and FIG. 6 (b) are illustrations for explaining the potentialeffect on the charged ray screen;

FIG. 7 is a graph of the relationship between the total counting rate(Hz) and background noise in the open counter along with changes in thevoltage supplied to the ray screen;

FIG. 8 is a graph of the counting rate (Hz) and background noise in theopen counter improved by the use of the ray screen structure;

FIG. 9 is a graph of the counting rate (Hz) and background noise in theopen counting device improved by the use of a chrome oxide layerstructure on the surface of grids;

FIG. 10 is the device in sectional view which is improved by the use ofa center guard structure further to enable nearby detection; and

FIG. 11 is another example of the center guard structure in sectionalview.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1A, a ray source unit 4 uses a heavy hydrogen lamp 11, a slit 12and a spectroscope 13 in which rays of the source 11 pass through theslit 12, and are then dispersed by the spectroscope 13 in to ultravioletrays of a predetermined wavelength which are irradiated on the surfaceof the subject 2 as an ultraviolet beam 3 through the slit 12.

Setting the subject 2 on which the ultraviolet beam 3 irradiates in avertical direction from the ray source unit 4, the photoelectrondetector 1 is placed so as to have its electron entrance gate 9 directedobliquely face to face with the surface of the subject 2 on which thebeam irradiates. The ray screen 14 which is attached at the detector 1to cross between the subject 2 and the detector 1 shelters the detectorfrom scattered rays 10, so that the inner part of the electron detector1 is not irradiated by the scattered rays from the subject 2 through theelectron entrance gate 9.

The ray screen 14 uses, for example a plate having a portion 14a cut offlike semi-circle through which the ultraviolet beam passes (see FIG. 4),or a plate having a cut off hole 14b for passing the beam therethrough(see FIG. 5). It also uses a plate having no portion cut off like 14a or14b to pass the beam but interrupt the scattering rays.

To the ray screen 14, a DC voltage Vc is supplied from an electricsource 15. The charged ray screen 14 makes emitted low-energy electronsor anions drift forward to the anode A5 which is charged to 3.5 KV,preventing them from being repelled outside. The potential which issupplied to a specimen will be controlled to be a voltage equal to thevoltage Vc of the ray screen voltage source 15, so that the electricfield thus produced at the ray screen 14 makes so as to cause low energyelectrons emitted from the specimen 2 to drift to the anode A5 byoverriding the ray screen. If the electric potential of the ray screen14 is equal to that of the subject 2 which is grounded as shown in FIG.6(a), photoelectrons or anions with low energy, which were emitted fromthe surface of the subject 2 upon irradiation of the ultraviolet beam 3or made accompanyingly by the emitted photoelectrons, are being inducedforward to the photoelectron open-counter 1 as is, but obstacled by theray screen 14 so as not to drift towards the anode A5. Thus, the exactcounting rate (Hz) was not always measured.

Referring to FIG. 6(b), the subject 2 is grounded and the ray screen 14is supplied with the Vc voltage inasmuch as remarkable effect will beconfirmed in FIG. 7 illustrating the relationship between the totalcounting rate S (Hz) and the background noise N (Hz) corresponding tothe change of the Vc voltage applied to the ray screen 14.

In the graph, the total counting rate S was measured at 3.94 KV of theanode A5 (Va), 100 V of the first grid G6 (Vg1) and 80 V of the secondgrid G7 (Vg2) regarding an A1 plate as the subject 2, and backgroundnoise N (Hz) was measured at -30 V of the second grid G7 (Vg2).

Analyzing the curve S increasing with higher voltage Vc and thesaturated curve between 16-20 V, it confirmed that the roundabout way oflow-energy electrons or anions could be made from the subject 2 to theanode A5 through the photoelectron entrance gate 9 in spite of thepresence of the ray screen 14. Meanwhile, the background noise N whichwas measured at -30 V of the second grid G7 (Vg2) was suppressed under10 cps independent from potential of the ray screen 14. The Vc voltageto the ray screen 14 is preferably selected from a range 16-20 V if thesubject is grounded. Under 10 V, emitted photoelectrons or anions wouldbe obstacled to drift towards the anode A5 by the low potential of theray screen 14.

Background noise is further suppressed by use of surface layerapplication on grids G6 and G7 which are positioned behind thephotoelectron entrance gate 9, and the surface layer is to have a workfunction larger than that for emitting photoelectrons by irradiation ofthe scattered rays from the subject, such that photoelectron emissioncan be prevented from the surface layer applied on grids having a largerwork function. Emission may be prevented by insuring that the layerthickness is larger than a range of from 30 to 400 A° for a large workfunction. This was verified by the fact that the mean free path oflow-energy electrons with energies less than 10 eV is on the order of afew micrometers. From the surface of metal making grids G6 and G7,photoelectron emission will be efficiently prevented because the surfacelayer has a work function larger than that of the grid metal.

Since grids G6 and G7 act to induce photoelectrons or anions towards theanode A5 in the electric field, the thickness of the surface layer willbe restricted to be able to keep the same function, because the functionof an efficient electric field is likely lost with a greater layerthickness. A thicker applied layer tends to make the electric fieldweak. The thickness should therefore be less than a few μm to preventphotoelectrons from being emitted from the grid metal by irradiation ofscattered rays. A graph of FIG. 9 shows that the suppression effectagainst background noise is less than 1/2 in comparison with FIG. 2having no layer application thereon. Accordingly, the S/N ratio may bepositively improved so that exact counting rate (Hz) will be morereliably measured out. Curves S1 and N1 are from the specimen 2 of FIG.3(a), and S2 and N2 are from FIG. 3(b). Chrome oxide was used to makethe layer or film, and it is useful to oxidize the surface of astainless grid. Furthermore, a layer or film of Nitride, or coating ofan organic compound or the like causes the same suppression function onthe surface of grid. When the surface layer formation is designed tohave a thickness of less than 1 μm, the dielectric affect will be easilycontrolled so as not to make total design complicated.

FIGS. 10 and 11 illustrate two examples of a center guard structure forsuppressing the background noise of scattered ray irradiation. A forwardend of optical fiber 101a into which an irradiation ray is supplied fromthe ray source 4 faces to the subject 2 passing through thephotoelectron entrance gate 9 of the open counter 1, which is guidedwithin a tubelike isolated holder 102 extending from the top to theoutside of the gate 9 of the open counter case 8. A level ring anode A5is horizontally supported by a conductor A6 around the isolated holder102, and the forward end 102a of the isolated holder is extended overthe gate 9 towards the subject 2 which is positioned at a predetermineddistance from a focussing lens 103 of the optical fiber 101. Anirradiation spot on the surface of the subject 2 may be controlledwithin a circle which is more or less than a μm order diameter.

Between the open end of the gate 9 and the peripheral edge of the holder102, the ray obstacle angle θ is defined by arrow A to repel theentrance of scattering rays from the subject 2, so that the forward end102a acts as a center guard structure. A contact tool 102b limits theupper measuring position secures nearby measurement for checking thethickness of the oxidized pattern layer on a VLSI chip on which theirradiation spot shall be focussed at a circle of a submicron orderdiameter.

In FIG.11, the center guard structure 102' is provided with a horn headas another example, in which a magnifying lens 103a is set at theforward end of the optical fiber 101 for forming an irradiation spotlarger than the optical fiber diameter. The ray obstacle angle θ isestablished between the horn head edge and the open end of the gate 9.This structure is useful for forming a controlled and larger spot, forexample, when a disfigurement such as scratches, abrasion or the like onthe surface of solids will be turned to surface information bycontinually irradiating the optical fiber ray beam on a predeterminedarea at an accurately controlled and magnified spot over the surface ofsubject 2 by a nearby measurement. In place of the horn head shape, adisk shape or an umbrella shape member are provided with the centerguard structure for respective needs.

The center guard structure 102 may be modified by replacing the forwardend by a metal or conductive tube to which the Vc is supplied. It ismost preferable to combine effects of the ray screen, the surface layeror film application and the center guard structure to effectivelysupress the background noise of scattered rays.

What is claimed is:
 1. A device for counting photoelectrons in aphotoelectron detector, which are emitted from a subject in response toirradiation by ray irradiation, in which the open photoelectron detectorconsists of photoelectron emission prohibition means for preventingphotoelectrons from being emitted from inner surfaces of the detectordue to rays scattered from the same subject; wherein said photoelectronemission prohibition means consists of a surface layer or film having awork function which is greater than that of parts of the detector, andwhich is applied to the parts from which photoelectrons are emitted dueto rays scattered from the subject.
 2. A device according to claim 1,wherein the photoelectron detector is arranged independent from anirradiation ray source which outputs an irradiation ray which is ledthrough an optical fiber to a point for making a spot over the surfaceof the subject to limit a scattering range on which the irradiation rayis scattered from the subject.
 3. A device according to claim 1, inwhich an optical device is provided with an optical fiber having aforward end for forming an irradiation spot corresponding tomeasurements of the subject and for limiting the scattering range.
 4. Adevice according to claim 2, in which an optical device is provided withan optical fiber having a forward end for forming an irradiation spotcorresponding to measurements of the subject and for limiting thescattering range.
 5. A device for counting photoelectrons in aphotoelectron detector, which are emitted from a subject in response toirradiation by ray irradiation, in which the open photoelectron detectorconsists of photoelectron emission prohibition means for preventingphotoelectrons from being emitted from inner surfaces of the detectordue to rays scattered from the same subject; wherein said photoelectronemission prohibition means consists of a ray screen for preventingscattered rays from being radiated to the detector from the subject, towhich is supplied a voltage to produce an electric field for deflectingaway from the detector the subject emission photoelectrons or anionsformed with the same emitted photoelectrons towards the detector.
 6. Adevice according to claim 5, wherein the photoelectron detector isarranged independent from an irradiation ray source which outputs anirradiation ray which is led through an optical fiber to a point formaking a spot over the surface of the subject to limit a scatteringrange on which the irradiation ray is scattered from the subject.
 7. Adevice according to claim 5, in which an optical device is porovidedwith an optical fiber having a forward end for forming an irradiationspot corresponding to measurements of the subject and for limiting thescattering range.
 8. A device according to claim 6, in which an opticaldevice is provided with an optical fiber having a forward end forforming an irradiation spot corresponding to measurements of the subjectand for limiting the scattering range.
 9. A device for countingphotoelectrons in a photoelectron detector, which are emitted from asubject in response to irradiation by ray irradiation, in which the openphotoelectron detector consists of photoelectron emission prohibitionmeans for preventing photoelectrons from being emitted from innersurfaces of the detector due to rays scattered from the same subject;wherein said photoelectron emission prohibition means consists of a rayscreen for preventing scattered rays from being radiated to the detectorfrom the subject, said ray screen being formed with a large workfunction layer or film on a surface of the screen which would emitphotoelectrons when irradiated with the scattered rays from the subject.10. A device according to claim 9, wherein the photoelectron detector isarranged independent from an irradiation ray source which outputs anirradiation ray which is led through an optical fiber to a point formaking a spot over the surface of the subject to limit a scatteringrange on which the irradiation ray is scattered from the subject.
 11. Adevice according to claim 9, in which an optical device is provided withan optical fiber having a forward end for forming an irradiation spotcorresponding to measurements of the subject and for limiting thescattering range.
 12. A device according to claim 10, in which anoptical device is provided with an optical fiber having a forward endfor forming an irradiation spot corresponding to measurements of thesubject and for limiting the scattering range.